1. Field of the Invention
The present invention relates to a circuit board which is mounted with groups of elements each including an active element and a passive element. More particularly, it relates to a circuit board which ensures an endurance reliability and a heat radiation performance over fixed levels, and which can be configured to a small size.
2. Related Art
Heretofore, a circuit board for a power source apparatus (hereinbelow, termed the “powermodule” on occasion), for example, the power module of an inverter which is mounted on a system for the drive control of an electric vehicle, or the like, has been often configured as stated below (refer to Patent Documents 1 and 2).
Such a power module is provided with a board (hereinbelow, termed the “arm”) which is so configured that one of two conductor layers respectively disposed on the two surfaces of an insulating substrate, and a semiconductor chip including a switching element, etc. are joined by a first solder portion, and a heat radiation plate. The other of the two conductor layers disposed on the insulating substrate of the arm, and the heat radiation plate are joined by a second solder portion.
[Patent Document 1] US 2002/0047132
[Patent Document 2] US 2006/0157862
A power module which ensures an endurance reliability and a neat radiation performance over fixed levels and which is configured to a small size, has been demanded in recent years. However, any power module capable of satisfactorily complying with the demand has not been found among power modules including ones disclosed in Patent Documents 1 and 2.
By way of example, as a test for verifying the endurance reliability of a general power module, there is a temperature cycle test in which the power module is repeatedly exposed under both the environments of a low temperature and a high temperature.
From the result of the temperature cycle test, it has been known that thermal stresses are applied between an insulating substrate and a heat radiation plate, that cracks appear at the fillet parts of a second solder portion around the corners of the insulating substrate, and that the cracks evolve gradually from the corners toward the center of the insulating substrate. It has also been known that, when such cracks evolve to arrive directly under a semiconductor chip, the heat radiation performance for heat generated from the semiconductor chip worsens.
Nevertheless, in the power module disclosed in Patent Document 1, a plurality of semiconductor chips having an identical function are respectively mounted near the corners of a single insulating substrate. Therefore, the power module disclosed in Patent Document 1 undergoes the influence of the cracks on the plurality of semiconductor chips earlier, and ensuring an endurance reliability and a heat radiation it performance degrades correspondingly.
Incidentally, Patent Document 2 discloses a technique in which a single semiconductor chip is mounted on substantially the central part of an insulating substrate. When the power module of Patent Document 2 adopting such a technique is compared with the power module of Patent Document 1, the time at which the semiconductor chip undergoes the influence of cracks seems to delay in correspondence with the existence thereof at the central part.
The “semiconductor chip” termed in Patent Document 2, however, signifies the whole element group which includes an IGBT (Insulated Gate Bipolar Transistor) being an active element that performs an active operation such as a switching operation, and an FWD (Free Wheeling Diode) being a passive element that does not perform such an active operation.
Accordingly, even when the whole element group is merely mounted on the central part, there is a drawback as stated below. If the insulating substrate is small, eventually part of the element group will exist near the corner of the insulating substrate, and the early influence of the cracks will be unavoidable. Herein, in order to avoid the early influence of the cracks (in order to delay the time of the influence), the size (area) must be made much larger than the size of the whole element group being the whole semiconductor chip, and this is contradictory to the requirement of the reduction of the size.
Besides, by way of example, the solder thickness of the second solder portion is not constant over the whole circuit board, and a phenomenon in which the arm inclines, sometimes occurs. In such a case, a solder thickness directly under the semiconductor chip differs every power module, with the result that also a thermal resistance differs. Concretely, in relation to the thermal conductivity of a solder and the solder thickness, the increase of the solder thickness leads to the tendency that the thermal resistance enlarges correspondingly, so a heat radiation property worsens. Accordingly, in the power module in which the solder thickness directly under the semiconductor chip is large, the thermal resistance enlarges correspondingly, and it is apprehended to exert influence on the heat radiation performance.
Besides, the thermal stress which incurs the cracks stated above depends on the solder thickness of the second solder portion, and it becomes larger as the solder thickness becomes smaller. Therefore, in the power module in which the solder thickness directly under the semiconductor chip is small, the possibility of the appearance of the cracks enlarges correspondingly, and the solder thickness of the second solder portion is apprehended to exert influences on the endurance reliability and the heat radiation performance.
Nevertheless, the semiconductor chip is not arranged in consideration of such a solder thickness of the second solder portion, in any of the prior-art power modules including ones disclosed in Patent Documents 1 and 2. The solder thickness of the second solder portion is therefore apprehended to exert influences on the endurance reliability and the heat radiation performance.
Incidentally, the demand of ensuring the endurance reliability and the heat radiation performance over fixed levels and being configured to a small size is required, not only in the power modules, but also generally in a circuit board which is mounted with an element group including an active element and a passive element.